1. Field of the Invention
The present invention relates to a webbing retractor with a locking mechanism which suspends rotation of a take-up shaft and thereby prevents a webbing from being drawn out.
2. Description of the Related Art
Various types of locking mechanisms for the webbing retractor are known. One example is disclosed in Japanese Utility Model Laid-Open No. 62-13751. In this webbing retractor, when an acceleration sensor is operated, a rotary wheel starts rotating in the direction in which a webbing is drawn out in synchronism with a take-up shaft. As the rotary wheel is rotated, a friction spring, which is wound around the outer peripheral surface of the rotary wheel in a compressed state by means of its own elasticity, rotates together with the rotary wheel, operating a claw member. As a result, the claw member is brought into engagement with a ratched wheel coaxially fixed to the take-up shaft, and rotation of the take-up shaft in the direction in which the webbing is drawn out is suspended. In this locking mechanism, the operation starting time of the claw member is precisely set relative to the pawls of the ratchet wheel in order to enable the operating claw member to be engaged with the ratchet wheel quickly and thereby ensure reliable prevention of the drawing-out of the webbing.
However, in this locking mechanism, since the claw member is operated due to a frictional force between the rotary wheel and the friction spring, the force with which the claw member is operated is not large, necessitating that various considerations be taken in order to operate the claw member smoothly.
Hence, the friction spring is wound around the rotary wheel in a clocking fashion so as to ensure reliable engagement between the friction spring and the rotary wheel while the locking mechanism is operating. Furthermore, relative movement between the friction spring and the rotary wheel is allowed during the abnormal operation, e.g., when the webbing is abnormally drawn out.
In this way, the force with which the claw member is operated is increased.
However, when the force with which the claw member is operated is increased, the friction between the friction spring and the rotary wheel increases, requiring that the webbing is abnormally drawn out with a large force. Thus, the friction which is generated during the normal operation is inconsistent with that which is generated during the abnormal operation.